I have an M312, and I think it's nice mixer, but I haven't used it all that much,...
so I don't have any specific comments about use. The B model has a few extra features over the std model, but I don't have a spec sheet on what those features may be.
The M312 is a solid, complex board, with good sound quality & good features. It's a bit more complex than the M30, so it may take a little longer to grasp all the signal flow & routing options. Things like having to route TAPE RTN through the LINE IN on each MAIN channel strip [for mixdown] may seem a little tricky at first, and having the manual helps a lot. Refer to the manual when necessary.